The anaerobic digestion process is a two stage process involving hydrolyzing, acid forming, and methane forming bacteria. In the first stage, the hydrolyzing and acid forming bacteria convert volatile solid materials to soluble compounds and then to various volatile fatty acids such as acetic acid, generating carbon dioxide and hydrogen in the process. In the second stage, the methane forming bacteria convert the volatile fatty acids to methane and carbon dioxide, and also combine carbon dioxide with hydrogen to form methane and water.
With many wastes, the waste components are largely soluble, and the conversion of volatile solids material to soluble compounds is not critical.
The overall reaction is as follows: ##STR1## The proton acceptor may be NO.sub.3 -, which is reduced to NO.sub.2 -, N.sub.2 O, or N.sub.2. If SO.sub.4 -- is present, it will be reduced to H.sub.2 S.
Depending on the waste composition, the rate limiting step in the reaction may be either volatile solids conversion or methane production. Many industrial and food processing wastes contain significant soluble compounds but relatively less volatile solids materials. The rate limiting step in the reaction of these wastes is the methane production step. The methane forming bacteria are sensitive to temperature, food, and pH conditions so that these conditions should be optimized so as to increase the overall rate of the process. Thus, anaerobic digestion is usually carried out at 25.degree.C.-40.degree. C., preferably at about 35.degree. C. Anaerobic digestion can also occur from 15.degree. C. to 60.degree. C. The pH of the process at steady state is normally around 6.8, although it can range from 6.0 to 8.0. Large changes in system temperature or pH can deactivate the methane-forming bacteria, but if the system returns to ideal conditions these bacteria will become active again.